The present invention relates to the field of implantable medical devices and, in particular, to a cardiac stimulation device employing a CFx battery for improved performance and a system for monitoring charge level of the battery.
Implantable medical devices are typically battery powered devices that are implanted within the patient""s body to have therapy available to the patient on a continuous basis. Battery failure is a particular problem with these devices as replacement of batteries often requires invasive surgical procedures. One particularly common type of implantable medical device is an implantable cardiac stimulation device.
Implantable cardiac stimulation devices, such as pacemakers and implantable cardioverter/defibrillators (ICD""s), are employed to monitor cardiac activity and to provide therapy for patients with a variety of heart arrhythmias. Typically, these devices include sensors, that sense heart function and physiological parameters, and waveform generation and delivery systems, that provide electrical waveforms to the heart to correct arrhythmias and to ensure that more proper function of the heart is maintained. As the devices are implanted in a patient, it is desirable that the devices be as small and lightweight as possible in order to minimize impact on the patient.
Implantable cardiac stimulation devices are typically provided with batteries to power the monitoring and therapy delivery circuits. Due to the size constraints, the batteries used in implantable cardiac stimulation devices must be very small in size and yet able to provide power over a long period of time. Once the device is implanted, replacement of batteries typically involves invasive surgery. Hence, there is a strong desire to have small batteries that can provide significant power output to power the implantable device for extended periods of time. Known pacemaker devices typically use lithium iodine (LiI), commonly referred to as lithium batteries. Lithium batteries offer relatively high energy storage density and have known, predictable discharge characteristics.
While lithium batteries are commonly used for implantable cardiac stimulation devices, these batteries require additional circuitry that degrade device performance. Specifically, the performance characteristics of these batteries often require that additional circuitry be added to the device, thereby resulting in consumption of limited space in the implantable device and also consumption of limited power, or this additional circuitry has performance characteristics that limit the useful life of the implantable device.
For example, FIG. 1 illustrates a high-level conventional pacemaker circuit diagram of the prior art. Lithium batteries are typically not capable of providing pacing pulses at increased energy levels. As is shown in FIG. 1, a typical lithium battery and a decoupling capacitor are often connected in parallel to address this problem. The decoupling capacitor is used to accumulate electrical charge between pacing pulse events to enable the pacemaker to periodically deliver a pulse of energy at a greater rate than a lithium battery is capable of providing directly. The decoupling capacitor is continuously charged by the lithium battery and discharged upon a pacing event.
However, known implementations of the decoupling capacitors of the requisite electrical properties occupy a large fraction of the overall volume of the pacemaker device. As pacemakers shrink in size due to product refinement, the size of the capacitor is becoming an increasingly larger proportion of the total pacemaker volume and is presenting a limitation to further reduction in the size and weight of pacemaker devices.
FIG. 1 illustrates another aspect of known pacemaker designs, in particular, a voltage tripler that is part of the control circuitry for the pacemaker. The control circuitry performs the basic timing and monitoring functions of the device and delivers the pacing pulses to the patient""s heart. The voltage tripler increases the voltage delivered by the lithium battery in order to provide a sufficient potential for effectively stimulating the heart. A lithium battery will have an open circuit voltage of approximately 2.7 VDC in a fully charged condition and approximately 2 VDC near the end of its life and thus requires a voltage tripler to generate the more than 5 VDC required for an effective pacing pulse. However, the voltage tripler is a source of overall system inefficiency as each voltage multiplication incurs some degree of loss.
A further drawback to the lithium battery will be apparent considering the output voltage characteristics illustrated in FIG. 2, which shows a typical voltage vs. charge delivery graph for typical lithium batteries. Multi-chamber pacing is a feature of many pacemaker systems and comprises supplying pacing stimuli to two different sites in the heart as opposed to pacing a single site. Typical parameters for a single-chamber pacing system with a lithium iodide battery at beginning of life would be an open circuit voltage of approximately 2.7 V (8.1 V after the voltage tripler) with an internal battery impedance of 300 xcexa9 and a single lead of 500 xcexa9 impedance. The voltage across the lead is regulated to be approximately 5 V and the pulsed current would be approximately 10 mA. Pulses are approximately 1 ms in duration and are applied every second, thus drawing a time averaged current of approximately 10 xcexcA.
Similar multi-chamber pacing to two sites through two 500 xcexa9 leads connected in parallel would draw a current pulse of approximately 20 mA and a time average current of 20 xcexcA. However, as a lithium battery is discharged, the open circuit voltage drops while the internal resistance increases. After delivering approximately 900 mA-h, a typical lithium battery""s output voltage decreases to approximately 2.4V and the internal impedance increases to approximately 10 kxcexa9. Under these parameters, delivering to two leads with 20 xcexcA average current pulls the battery output voltage down to 2.2 V (2.4 Vxe2x88x9220E-6 Axc3x9710E3 xcexa9) and thus approximately 6.6 V after the tripler. These battery conditions give marginal performance even with the voltage tripling.
With further use, i.e., further discharge of the lithium battery, the open circuit voltage continues to decrease and the ESR continues to increase to approximately 30 kxcexa9 at EOL. Thus, while a lithium battery in this condition still has considerable charge remaining, the internal impedance and voltage at which the charge is available render a lithium battery unsuitable for continued multi-chamber pacing. Because of this factor, approximately 30-50% of the total typical lithium iodide battery""s capacity is not usable and is wasted.
Thus, the typical lithium/lithium iodine battery currently in use in many implantable cardiac stimulation devices generally requires additional components to deliver the power needed to provide therapy and also has a limited life span in some implementations. Limited life span, of course, requires more periodic follow up and also requires more frequent replacement of the device. As stated above, more frequent replacement of the device is undesirable as it typically requires invasive surgical procedures.
A further difficulty that occurs with lithium/lithium iodine batteries in implantable medical devices is that the internal configuration of the battery often limits telemetry transfer rates. The power that the lithium battery provides is generally not sufficient to support data transfer rates from implantable cardiac stimulation devices that are in excess of approximately 8 Kbit of data. Typically, the decoupling capacitors are limited to only providing sufficient power to source the pacing pulses but do not have the capacity for providing sufficient charge to maintain voltage during a multi-minute, high speed transmission. This relatively low rate of data transfer therefore requires longer download periods to obtain data out of the implantable device which can be very inconvenient for the patient and the treating medical professional as well as consuming additional limited power from the battery.
Other battery technologies exist that may have application in implantable medical devices, however, these technologies have not generally been used due to implementation problems. One such technology is lithium-carbon monoflouride (LiCF1.1), typically referred to as CFx batteries. CFx batteries have some desirable characteristics that show promise for use in implantable medical devices, such as pacemakers. Generally, CFx batteries have twice the mass energy density as lithium batteries and can thus provide significantly more electrical energy as lithium batteries of similar weight. Moreover, the performance characteristics are comparable to lithium based batteries.
However, the use of improved battery technologies, such as CFx batteries, in critical applications, such as implantable cardiac stimulation devices, has been limited by an inability to determine approaching end of life of the battery accurately and efficiently. In applications such as pacemakers and ICDs, it is imperative that the device be replaced prior to the battery failing. Battery failure will result in the device being unable to provide therapeutic simulation to the heart which can further result in catastrophic consequences for the patient.
CFx batteries provide a very stable voltage output throughout their effective life. As a consequence, CFx batteries do not exhibit output characteristics that are easily modeled to predict the end of life. Basically, these batteries generally maintain a substantially constant output voltage until the end of life and then their power output drops off very precipitously. This makes detection of approaching end of life of the battery extremely difficult. As a consequence, the application of CFx batteries to patient critical applications, such as pacemakers, ICDs and the like, has been extremely limited.
From the foregoing, it will be appreciated that there is a need for improved batteries for implantable medical devices such as pacemakers and ICDs. To this end, there is a need for a mechanism for detecting end of life of battery technologies, such as CFx batteries that have relatively constant output voltages over the length of their life to allow the use of these batteries in patient critical applications.
The aforementioned needs are satisfied by the implantable medical device of the present invention which, in one aspect, comprises an implantable medical device that is adapted to provide therapy to a patient that includes a battery that has a relatively constant output voltage until end of life and a voltage monitor that is monitoring the output voltage of the battery to thereby predict the end of life of the battery to enable the implantable medical device to signal the need of replacement of the battery.
In one particular implementation, the implantable medical device comprises an implantable cardiac stimulation device that includes a microcontroller and a battery, such as a CFx battery, that provides a constant output voltage, e.g., a variation of less than 200 millivolts over 1800 Milliampere hours of operation until end of life of the battery. At end of life of the battery, the supply voltage drops off rather precipitously, e.g., from approximately 2700 millivolts to 700 millivolts in less than 400 Milliampere hours.
The use of a voltage monitor permits the use of batteries, such as CFx type batteries, that have reduced internal resistance, also known as equivalent series resistance, thereby reducing the need for extra components such as de-coupling capacitors in pacemaker applications. Moreover, the reduced internal resistance also permits higher data transfer rates during telemetry and also reduces the need for voltage triplers as less energy of the battery is being absorbed by the battery itself.
In one particular implementation, the voltage monitor includes a band gap reference and a precise A/D, e.g., a 12 bit A/D that compares the battery voltage to the band gap reference voltage. This configuration is capable of monitoring the output voltage of the battery to within 1.0 millivolt resolution. The A/D provides a digital output signal to the microcontroller and the microcontroller evaluates the digital output signal to determine if the battery is approaching end of life. In one aspect, the microcontroller evaluates the signal to first determine a predicted end point of the battery and then evaluates the subsequent signals to determine if the output of the battery is approaching the predicted end point.
In one implementation, the microcontroller determines if the battery is approaching end of life by ascertaining a beginning of life voltage of the battery and then determines that end of life is approaching when the battery output voltage has fallen over time back to the beginning of life voltage. In another implementation, the microcontroller determines that end of life is approaching by periodically measuring the output voltage and determining when a peak output voltage has occurred. When measuring voltage, the voltage may vary slightly due to the current being drawn from the battery at any given time. To address this in one implementation, the battery voltage is periodically measured over a pre-selected period of time, e.g., once a day for a week, and a rolling average is maintained. The rolling average can be calculated daily such that the effect of periodic variations in the daily measurements are reduced. The end of life point of the battery is then predicted by determining when the output voltage of the battery has decreased a pre-selected quantity from the peak voltage. In yet another implementation, the end of life point of the battery is approximated by calculating a rate of consumption of the stored battery power and then determining a end of life point at the rate of consumption.
Each of the above-implementations can be used in conjunction with others to determine a correlated approaching end of life point for the battery. Once an end of life point is determined for the implantable medical device, the microcontroller of the implantable medical device can set a register indicating that the end of life has occurred or can otherwise enable an annunciator to advise the patient that it is time to seek replacement of the battery.
The use of such a monitoring system enables the use of improved batteries, such as CFx batteries in patient critical applications. The use of these types of batteries can result in more efficient power consumption, improved data transmission and the like. These and other objects and advantages of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.